An HEV is a vehicle that has a propulsion system that consists of at least one electric motor or electric machine in combination with at least one other power source. Typically, the other power source is a gasoline or diesel engine. There are various types of HEVs depending on how the electric motor(s) and other power source(s) are combined with one another in order to provide propulsion for the vehicle, including series, parallel and compound HEVs.
Various hybrid powertrain architectures are known for managing the input and output torques of various propulsion systems in HEVs, most commonly internal combustion engines and electric machines. Series hybrid architectures are generally characterized by an internal combustion engine driving an electric generator which in turn provides electrical power to an electric drivetrain and to an energy storage system, comprising a battery pack. The internal combustion engine in a series HEV is not directly mechanically coupled to the drivetrain. The electric generator may also operate in a motoring mode to provide a starting function to the internal combustion engine, and the electric drivetrain may recapture vehicle braking energy by also operating in a generator mode to recharge the battery pack.
Parallel HEV architectures are generally characterized by an internal combustion engine and an electric motor which both have a direct mechanical coupling to the drivetrain. The drivetrain conventionally includes a shifting transmission to provide the necessary gear ratios for wide range operation.
Electrically variable transmissions (EVT) are known which provide for continuously variable speed ratios by combining features from both series and parallel HEV powertrain architectures. EVTs are operable with a direct mechanical path between an internal combustion engine and a final drive unit thus enabling high transmission efficiency and application of lower cost and less massive motor hardware. EVTs are also operable with engine operation mechanically independent from the final drive or in various mechanical/electrical split contributions (i.e. input split, output split and compound split configurations) thereby enabling high-torque continuously variable speed ratios, electrically dominated launches, regenerative braking, engine off idling, and two-mode operation.
As noted, such complex EVT HEVs utilize one or more electric machines and require advanced energy transmission, conversion and storage systems to supply electrical energy to and receive and store electrical energy from these machines, and would typically comprise, for example, at least one electric machine, power inverter module, power bus, energy storage device, such as a battery, as well as various control electronics, control algorithms and other associated items. The energy storage system (ESS) may comprise any suitable energy storage system that is adapted for high-density energy storage, including a battery, ultracapacitor, or other high-density energy storage device. As used herein, reference to a battery includes not only a single battery, also includes any combination of single or multiple batteries, or cells thereof, into a battery pack or array, or a plurality of battery packs or arrays. This invention is particularly suitable for use in a parallel array of battery packs, each of which comprised a plurality of batteries. As used herein, the term battery generally refers to any secondary or rechargeable battery, but those comprising lead/acid, nickel/metal hydride (Ni/MH, or Li/ion or polymer cells are preferred.
Given the dynamics associated with operation of an HEV, particularly the constant flow of energy into and out of the energy storage device, the battery plays a critical role in the operation of these vehicles. The critical role of the battery in these vehicles imposes a number of requirements on the battery performance, including both operational and service life requirements.
Significant attention has been given to maintaining the operational performance of batteries used in HEV applications. Particular attention has been given to various aspects of maintaining the battery pack state of charge (SOC). The SOC is defined generally as the ratio of the residual charge in a battery relative to its full charge capacity. Various hardware and software control strategies have been adjusted for determining and maintaining the SOC of the battery.
While understanding and maintaining the SOC of the battery is critical to its performance in HEV applications, it is not the only important characteristic of the battery. Another critical characteristic of batteries used in HEV applications is the useful life of the battery or battery pack. For example, it is known that secondary batteries, such as Ni-MH batteries, have limited amp-hour throughput that defines their useful service life. The anp-hour throughput or capacity of the battery is the integral of the energy flowing through the battery as a function of time as it is constantly charged and discharged in service.
While it is critical to manage various aspects of the ESS of an HEV as described above, it is also necessary to ensure certain aspects of vehicle performance, such as the vehicle launch characteristics. Vehicle launches are generally associated with starting the motion of the vehicle from a stop, and may be characterized by the speed of the vehicle and its required torque output at any given point during operation of vehicle (i.e., no or low speed and relatively high torque). However, launch conditions may also exist during other periods of vehicle operation, such as acceleration from a low-speed interval, or seeking to maintain or increase speed while negotiating an incline. Therefore, it is desirable to develop control algorithms for vehicle operation which ensure the management and protection of the ESS, particularly the battery, while at the same time ensuring that the ESS, including the battery, may be fully utilized to ensure optimum vehicle performance under launch conditions.